1. Field of the Invention
The invention relates to a method and apparatus for uniformly electrostatically charging an electrophotographic film.
2. Description of the Prior Art
In electrophotography, it is common to apply a uniform electrostatic charge to the surface of a photoconductive layer. The charge in selected areas is then dissipated in a pattern by exposing the surface to a light image to form an electrostatic latent image. The latent image is then rendered visible by applying thereto finely divided electrostatically charged developer particles which adhere to the surface by electrostatic attraction. Permanent visible images can be obtained, for example, by using thermoplastic developer particles which are heat fused to the photoconductive layer.
Charging is conventionally accomplished by exposing the surface of the photoconductive layer to a corona source, the polarity of which is chosen to produce the desired results upon the particular photoconductive layer being charged. The corona source commonly consists of one or more fine wires postioned close to the photoconductive layer. When a potential on the order of 3,000 volts to 10,000 volts is applied to the wire or wires, a corona is generated and ions are attracted to and deposited on the surface of the photoconductive layer to produce the charge thereon.
Production of negative corona, which is desired for most applications, with a fine wire or wires presents problems not encountered in the production of positive corona. The generation of positive corona appears as a continuous uniform visible sheath surrounding the wire, whereas the negative corona has a tendency to concentrate at discrete points along the wire, appearing and disappearing at different sites.
In electrophotography, superior recordings are obtainable only when very uniform electrostatic charges are established over the entire area of the electrophotographic recording medium before imaging. When there is relative movement between the corona generating elements and the electrophotographic recording medium, any non-uniformity in electrostatic charge which might have been established on the recording medium is substantially reduced, providing such relative motion is parallel to the recording medium and perpendicular to the axis of the corona generating elements. However, when the corona generating elements and the recording medium are substantially stationary, there is a tendency under some operating conditions to provide such non-uniformity of charge as to result in a variable intensity recording. This effect has been somewhat decreased by applying alternating or pulsating potential superimposed on a fixed DC voltage to the corona generating elements. However, even with such excitation there is a tendency to provide recordings having streaks parallel to the corona generating wires.
Electrophotographic recording is becoming popular for storing the ever-increasing volume of documentary material which is presently being generated. It has become particularly desirable to reduce the size of such documents by recording them on microfilm or microfiche. Apparatus for this purpose are conventionally capable of selectively imaging a predetermined portion of the film for each document. One particular apparatus for recording documents on microfiche is described in the previously referred to and commonly assigned U.S. Patent application Ser. No. 349,452, filed Apr. 9, 1973, in the name of Frank C. Gross. In operation of that apparatus, microfiche is positioned to place the predetermined frame on the optical axis of the lens system through which the image is directed and focused onto the fiche. Just prior to directing the image onto the fiche, the photoconductive layer of the fiche is charged by means of a corona discharge produced at an electrode positioned close to the microfiche on the lens axis. Because the film will also be imaged in this same position, it is not convenient to move the film with respect to the electrode during charging, or vice versa.
The apparatus described in U.S. Patent application Ser. No. 349,452 includes a lens module in which is positioned the lens and the corona discharge electrode. Because of confinement of components problems within the apparatus, the lens module size is limited. For example, the opening in the lens module against which the microfiche is placed is limited to the size of the frame to be charged. Also, it is desirable for the corona generating element to be subjected to little confinement to permit more uniform charging of the film. However, the restrictions on lens module size limits the space available for the element. Thus, the ratio of electrode confinement dimensions to corresponding module opening dimensions is limited. Furthermore, because of lens module size limitations, the spacing of the electrode from the film to be charged is limited to a closer spacing than is desired for uniform charging. All of these size limitations make charge uniformity difficult to obtain in this apparatus.
In the past, attempts to produce uniform electrostatic charging included such methods as moving the electrophotographic film with respect to the corona discharge electrode and subjecting the electrophotographic film to a high voltage corona for a sustained period of time. In practicing the latter method, excessive charging could occur, which would break down the photosensitivity of the film. Controls to prevent excessive charging of the film have included the use of shields usually grounded or biased, to attract the major portion of the ionized gases. While many of these attempts may have been suitable for the particular circumstances for which they were designed, they have not been satisfactory for all situations, particularly where no relative movement between the corona discharge electrode and electrophotographic film is possible and where the corona electrode is placed on the optical axis of the imaging system.
Particular disclosures of prior art corona charging are as follows. U.S. Pat. No. 2,879,395 to Walkup discloses a corona charging device in which high voltage AC potential is applied between a plurality of wire, corona discharge electrodes and control electrodes to create a corona discharge. The control electrodes, forming a grill, are positioned between the corona discharge electrodes and the photoelectric surface to be charged and are biased with a DC low voltage source to limit the charge applied to the photoelectric surface. U.S. Pat. No. 2,885,556 to Gundlach discloses a corona charging device which has a pair of wire, corona discharge electrodes, each positioned on opposite sides of the surface to be charged. High voltage AC potential is applied to each electrode to produce coronas of opposite polarity and a low voltage DC potential is connected to a shield in the vicinity of each corona electrode. The low voltage DC potential applied to each shield is of the opposite polarity to that applied to the other shield. The corona charging device disclosed in both the Walkup patent and the Gundlach patent requires relative movement during charging between the corona generating electrodes and the surface to be charged.
U.S. Pat. No. 3,307,034 to Bean discloses a corona charging device which employs a pair of wire, corona discharge electrodes placed on the sides of an imaging optical axis to avoid interfering with the imaging. A high voltage AC potential is applied to both of the corona electrodes with the end of the potential applied to one of the electrodes being opposite to the end applied to the other. High voltage DC biasing potential is applied to both electrodes to control the corona emission by producing corona of only a single polarity. The corona charging device disclosed in the Bean patent also employs shields which are DC biased. U.S. Pat. No. 3,237,068 to Sowiak discloses a corona charging device which employs multiple wire, corona electrodes which are connected to a high voltage DC potential source to produce a corona. The DC potential source is pulsed to reduce non-uniformity of the corona, particularly when negative corona is generated. The Sowiak patent describes the prior art as including AC potential superimposed upon a fixed DC potential to reduce the non-uniformity of negative corona production.
U.S. Pat. No. 3,708,661 to Hansen et al discloses a corona charging device which employs a needle, corona discharge electrode along with counterelectrodes to avoid overcharging the photoelectric surface. A high voltage DC potential is applied to the discharge electrode in the desired polarity to produce the corona. A high voltage DC potential is applied to the counterelectrodes in an opposite polarity to control with improved uniformity the amount of charge imposed on the photoconductive surface. U.S. Pat. No. 3,076,092 to Mott discloses a corona charging device which employs a plurality of wire, corona discharge electrodes connected to a high voltage AC potential source for production of the corona. A DC voltage is superimposed upon the AC voltage to bias the AC voltage so that it produces corona of only one polarity. The Mott apparatus requires relative movement during charging between the surface being charged and the corona generating electrode.
U.S. Pat. No. 3,390,266 to Epping discloses a corona charging device which employs a corona discharge electrode connected to a high voltage AC power source. A grounded metal bar shield is maintained in the vicinity of the corona electrode for production of the corona at the electrode, and a low voltage DC bias is applied between the corona discharge electrode and the surface to be charged to selectively draw charges of the desired polarity. One disclosed embodiment describes a screen grid positioned between the corona discharge electrode and the surface to be charged with the screen grid biased by a DC potential to a potential somewhere between the corona discharge electrode and the surface to be charged. The Epping device requires relative movement between the corona discharge electrode and the surface to be charged.
U.S. Pat. No. 3,332,396 to Gundlach discloses a corona charging device for purposes of development rather than imaging. The corona discharge electrode is connected to a high voltage AC power source which is DC biased to equalize the negative corona output with the positive corona output to create a neutral or zero voltage on the back side of a donor member. U.S. Pat. No. 3,492,476 to Germanos discloses a corona charging apparatus which employs a single wire, corona electrode connected to a high voltage DC power source. A shield in the vicinity of the corona electrode has an AC power source imposed upon it for purposes of improving uniformity of a negative corona generation in the electrode. Relative movement is required between the corona generating electrode and the surface to be charged. Examples of other patents disclosing corona generating devices are U.S. Pat. No. 3,335,273 to Walkup and U.S. Pat. No. 3,714,531 to Takahashi.
It a primary object of the present invention to provide a method and apparatus for uniformly electrostatically charging a photoelectric surface, particularly where that photoelectric surface is provided by a microfiche and the charging apparatus is undesirably limited in size. It is apparent that a number of disadvantages exist in the corona charging apparatus of the prior art. For example, some do not produce uniform charging over the entire surface to be charged. Also, some require complicated arrangements such as biased shields and screens and other electrical arrangements to improve charge uniformity. Additionally, many of them require relative movement between the surface to be charged and the corona discharge electrode, a step which is undesirable when the surface is to be charged and image-exposed in the same position. Furthermore, of those that are suitable for charging the photoelectric surface without relative movement between the surface and charging electrode, most are not capable of imaging immediately after charging while maintaining the charged electrode in a stationary position, because the charged electrode or electrodes interfere with the imaging.
The apparatus and method of the present invention is capable of overcoming all of these disadvantages.